24 MATER r A ME DIG A AND THERAPEUTICS. 



drugs and their action, it cannot be accomplished 

 without a thorough acquaintance with the physical, 

 chemical, and physiological properties of the ingredients 

 of the proposed compound. The mere appearance, 

 taste, and flavour of a mixture are important points 

 to be considered in ordering it. The chemical reactions 

 which may occur between the constituents must be 

 constantly kept in view. The prescriber may either 

 intend the constituents to remain chemically unchanged, 

 or arrange for the decomposition of one or more of 

 them, and the production of a new substance. Drugs 

 which decompose each other are said to be chemically 

 incompatible in the widest sense ; but the use of the 

 term is commonly restricted to instances in which the 

 result is an unexpected, inelegant, useless, or dangerous 

 compound. Thus, if it be desired to give a patient 

 chlorate of potash and hydrochloric acid, we say that 

 the undiluted acid is incompatible with the salt, 

 because chlorine is produced by their combination ; 

 but if it be intended to order a fresh solution of 

 chlorine in water, and the decomposition is deliberately 

 planned, the combination would not be considered in- 

 compatible. A list of incompatibles will be found 

 under the " characters " of the principal drugs. 



The prime consideration, however, will be the phy- 

 siological effect of the combination. This is very diffe- 

 rent in different cases. Each of the constituents may 

 be intended to produce an effect different from the 

 others ; or to have the same effect ; or one or more 

 ingredients may be introduced to modify the action of 

 the principal, that is, to correct some unpleasant, 

 dangerous, or otherwise undesirable influence which 

 it happens to possess, in addition to the influence which 

 we wish to secure. Such correctives are necessarily 

 physiological antagonists, and appear, therefore, to be 

 physiological incompatibles ; but it is for this very 

 reason that they are to be combined, because whilst 



